1. Field of the Invention
This invention relates generally to a water vapor transfer (WVT) unit for humidifying a cathode inlet airflow to a fuel cell stack in a fuel cell system and, more particularly, to a WVT unit for humidifying a cathode inlet airflow to a fuel cell stack in a fuel cell system, where the WVT unit employs a spiral-wound design for reducing packaging space.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
As is well understood in the art, fuel cell membranes operate with a certain relative humidity (RH) so that the ionic resistance across the membrane is low enough to effectively conduct protons. The relative humidity of the cathode outlet gas from the fuel cell stack is typically controlled to control the relative humidity of the membranes by controlling several stack operating parameters, such as stack pressure, temperature, cathode stoichiometry and the relative humidity of the cathode air into the stack.
As mentioned above, water is generated as a by-product of the stack operation. Therefore, the cathode exhaust gas from the stack will include water vapor and liquid water. It is known in the art to use a water vapor transfer (WVT) unit to capture some of the water in the cathode exhaust gas, and use the water to humidify the cathode input airflow. In one known design, the WVT unit includes flow channels defined by stamped metal plates and a membrane positioned therebetween. Water in the cathode exhaust gas flowing down the flow channels at one side of the membrane is absorbed by the membrane and transferred to the cathode air stream flowing down the flow channels at the other side of the membrane.
The WVT units used for a fuel cell system of the type discussed herein typically need to be compact, have a low pressure drop and have a high performance. Through fundamental model studies and testing, certain parameters for such a WVT unit have been identified. These parameters include use of a membrane material having a very high transfer performance, i.e., equivalent or better than Nafion 111, and a distance between the bulk gas and the surface of the membrane less than 0.5 mm, and preferably less than 0.33 mm, to reduce the wet gas phase mass transfer resistance resulting in a very small repeating distance enabling laminar flow and a low pressure drop.
One known design for fuel cell applications is a planar type WVT unit having repeating cells. In this design, the WVT unit includes two types of plates, i.e., wet plates and dry plates, that are alternately stacked with divider plates therebetween. However, the thickness of the divider plates in the repeating cell design contributes significantly to the thicknesses of the cells resulting in a larger and heavier assembly. Thus, it may be desirable to improve the divider plate design to reduce the size of the WVT unit.